Methods and devices for laser beam parameters sensing and control with fiber-tip integrated systems

ABSTRACT

A sensing method for in-situ non-perturbing measurement of characteristics of laser beams at the exit of the laser beam delivery fiber tips include measuring power of a laser beam transmitted through delivery fiber tip in fiber-optics systems. A sensing devices for in-situ non-perturbing sensing and control of multiple characteristics of laser light transmitted through light delivery fiber tips includes a fiber-tip coupler comprised of a shell with enclosed delivery fiber having a specially designed angle-cleaved endcap and one or several tap fibers that are specially arranged and assembled at back side of the endcap and other variations. Methods and system architectures for in-situ non-perturbing control of characteristics of laser beams at the exit of the laser beam delivery fiber tips include fiber-tip couplers and sensing modules that receive laser light from tap fibers, and systems for optical processing to enhance light characteristics suitable for in-situ measurement.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.16/411,299, filed May 14, 2019, titled “Methods and Devices for LaserBeam Parameters Sensing and Control with Fiber-Tip Integrated Systems,”the disclosure of which is incorporated by reference in its entirety.

FIELD

The disclosed technology pertains to sensing systems for in-situnon-perturbing measurement and control of characteristics of laserbeams.

BACKGROUND

Glass fibers and fiber-optics devices may be used to route, distribute,and deliver laser beams from single or multiple laser sources todifferent destinations, including devices, machines, and systems thatare located a distance from the laser source. The fiber-based devicesare also widely utilized for intentional manipulation of laser beamcharacteristics inside the fiber (e.g. laser light power, polarization,phase). Laser beams propagate in fibers and fiber-optics systems overcertain distances prior to being transmitted into air or other mediathrough a tip of the fiber end-section, which may be referred to hereinas the delivery fiber. The optical fiber itself may have a core andcladding areas having slightly different refractive index based on thematerial of their construction. Both the fiber core and cladding mayprovide guiding of laser light components known as modes, or guidedtransversal modes. The fibers that preserve propagation of a single-mode(SM) laser beam may be referred to herein as SM fibers. The SM fiberscan be made to also maintain, substantially unchanged, the polarizationstate of the propagating in fiber light. These fibers are referred to aspolarization maintaining (PM) SM fibers. In the so-called multi-modefibers, the fiber core is larger compared to the SM fibers ranging fromtens to hundreds of microns. These fibers may be known as large modearea (LMA) fibers and the multimode fibers guide either a few (few-modefibers) or a number of transversal modes that comprise laser lightinside the fiber and immediately after exiting the delivery fiber tip.The multimode fibers allow for much easier launching of light inside thefiber and can provide guiding of laser beams with significantly higherpower (up to and above 10 kilowatts) if compared with the SM fibers. Atthe same time SM and LMA fibers provide better quality laser beam (e.g.,a single-mode Gaussian shape beam) that has smaller divergence afterexiting the fiber delivery fiber tip, which may be advantageous andhighly desired for many applications.

Factors such as imperfections in fiber manufacturing, differentoperations with fibers (e.g., splicing of different fibers, fibermodification for stripping of high-order modes to maintain SM operationof a fiber system, tapering of fibers), environmental effects such asvibrations and temperature fluctuations, mechanical factors (e.g., fibertwisting and or bending), acoustical disturbances, and other factors maycause deviations in characteristics of the laser light that istransmitted from the delivery fiber tip. Examples of deviations from theexpected or desired laser beam characteristics may include, for example:(a) temporal fluctuations of the transmitted laser power, polarizationstate, piston phase that is associated with deviations in optical pathlength in fiber systems; (b) appearance of undesired laser irradiancecomponents such as high-order modes, light components coming through thefiber cladding (e.g., cladding light), laser light used as a pump infiber amplifiers and fiber lasers (e.g., residual pump light); (c)changes in the transmitted light characteristics that are associatedwith non-linear effects in fibers such as the stimulated Brillionscattering (SBS) and non-linear phase modulation; (d) changes in thetransmitted light that are related with external light that is receivedthrough the fiber tip (e.g. back reflected light).

It may be advantageous for such deviations from the expected or desiredtransmitted light characteristics to be detected in situ.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings and detailed description that follow are intended to bemerely illustrative and are not intended to limit the scope of theinvention as contemplated by the inventor.

FIG. 1A is a schematic illustration of an exemplary approach for laserbeam sensing using external light tapping that utilizes an external beamsensing optical system;

FIG. 1B is a schematic illustration of an exemplary approach for laserbeam sensing using fiber-based light tapping based on fiber-integratedbeam splitter and/or fiber tap coupler;

FIG. 2A is a schematic illustration of external beam sensing using asmall tap mirror placed in the beam path;

FIG. 2B is a schematic illustration of external beam sensing utilizing abeam splitter to redirect a small portion of the transmitted beam into asensor;

FIG. 3 is an illustration of a fiber-tip coupler for in-situ sensing ofthe laser light transmitted through a delivery fiber tip, using anotional schematic of the fiber-tip coupler cross section along theoptical axis;

FIG. 4 is a notional schematic of a cross section along the deliveryfiber tip of FIG. 3;

FIG. 5 is a schematic view of an exemplary ray tracing-based analysis ofthe fiber-tip coupler of FIG. 3;

FIG. 6A is a front elevation view of an exemplary single-tapfiber-tip-coupler assembly based on multi-hole glass ferrule (MH-GF);

FIG. 6B is a side view of the single-tap fiber-tip-coupler assembly ofFIG. 6A;

FIG. 6C is a graph showing an exemplary set of measured and approximatedcalibration data that may be utilized for the transmitted laser powersensing using the MH-GF device of FIG. 6B;

FIG. 7 is a notional schematic illustrating a concept of an all-glasscapillary-integrated multi-tap (CIMT) fiber-tip-coupler sensing moduleshowing the CIMT cross-section in the plane of tips of delivery and tapfibers;

FIG. 8A is a front elevation view of an exemplary CIMT sensing deviceshowing the CIMT cross-section in the plane of tips of delivery and tapfibers;

FIG. 8B is a side view of the CIMT of FIG. 8A showing an end cup sectionand capillary end-section displaying the delivery and tap fibersentering the capillary;

FIG. 8C is a side view of the CIMT sensing device of FIG. 8A and FIG.8B;

FIG. 9A shows a graph of exemplary calibration data of the CIMT deviceof FIG. 8A obtained using measurements and numerical simulations to beutilized for sensing of the main beam quality;

FIG. 9B shows the CIMT cross-section in the plane of tips of deliveryand tap fibers with the distribution of the laser power received by the13 tap fibers in the form of a polar plot that is superimposed with sideelevation view of the tap fibers in FIG. 7.

FIG. 9C shows a graph of exemplary calibration data of the CIMT deviceof FIG. 7 utilized for sensing contribution of cladding and residuallight;

FIG. 9D shows a graph of exemplary calibration data of the CIMT deviceof FIG. 7 utilized for sensing of the transmitted laser power;

FIG. 10 is a general schematic of a fiber-tip-coupler sensing andcontrol (FTC-SC) system architecture;

FIG. 11 is a notional schematic of an exemplary FTC-SC system forreal-time control and stabilization (locking) of the transmitted mainbeam polarization state in a fiber-optics system;

FIG. 12A is a graph of exemplary experimental data illustrating abilityof the FTC-SC system of FIG. 11 for real-time stabilization of thetransmitted main beam polarization angle in a fiber-optics system basedon SM non-PM fibers and fiber-elements;

FIG. 12B is a graph of exemplary experimental data illustrating theability of the FTC-SC system of FIG. 11 for real-time polarizationextension ratio increase of the transmitted main beam in a fiber-opticssystem based on SM non-PM fibers and fiber-elements;

FIG. 13 is a notional schematic illustrating a method and control systemarchitecture for polarization locking and control of multiple laserbeams in fiber array laser systems using the fiber-tip-coupler sensingmodules; and

FIG. 14 is a notional schematic illustrating a method and control systemarchitecture for locking and control of piston phases of multiple laserbeams in coherent fiber array laser systems using the fiber-tip-couplersensing modules.

DETAILED DESCRIPTION

The inventors have conceived of novel technology that, for the purposeof illustration, is disclosed herein as applied in the context ofnon-perturbing (e.g., mitigating or minimizing any negative effects of)sensing of the characteristics of laser light that is transmitted offthe tip of delivery fiber in fiber-optics systems and utilization of thedisclosed sensing methods and devices for laser beam characteristicscontrol in fiber-optics systems. While the disclosed applications of theinventor's technology satisfy a long-felt but unmet need in the art offiber-optics systems, it should be understood that the inventor'stechnology is not limited to being implemented in the precise mannersset forth herein, but could be implemented in other manners withoutundue experimentation by those of ordinary skill in the art in light ofthis disclosure. Accordingly, the examples set forth herein should beunderstood as being illustrative only, and should not be treated aslimiting.

Sensing of the laser light characteristics at the delivery fiber tipexit may be advantageous for several reasons including but not limited:(a) quality evaluation of the transmitted laser light; (b) prevention ofcatastrophic events leading to fiber system partial or even fataldamage, such as fiber tip contamination, growth of non-linear effectsabove acceptable threshold, appearance of back reflected light ofunacceptably high power level; (c) active or adaptive control of thelaser beam characteristics to either mitigate the undesired deviationsin laser beam characteristics (e.g. stabilize the transmitted power,eliminate high-order modes, decrease phase noise, nonlinear effects), orto set one or several laser beam characteristic to a desired state, forexample, to provide desired mutual stabilization (e.g., locking) ofpiston phases or polarization states in the master-oscillator poweramplifier (MOPA) type multi-channel fiber and fiber array systems.

It may also be advantageous to provide a sensing system used for in-situdetection of the transmitted through fiber tip light that isundisruptive and doesn't perturb characteristics of this light.Implementations of the disclosed herein advanced methods and devices forin-situ non-perturbing sensing of multiple characteristics of laserlight transmitted through light delivery fiber tip will create newopportunities for a wide range of fiber optics technologies especiallyin the areas of high power fiber systems for material processing,defense and aerospace applications.

Conventionally, sensing of the laser beam characteristics infiber-optics systems may be performed using approaches such as thoseillustrated in FIGS. 1A and 1B. The approach shown in FIG. 1A, referredto as external light tapping, is based on splitting or tapping opticalpower of a laser beam (100.1) that is transmitted through a fiber tip(100.2) of the delivery fiber (100.3), from the fiber-optics system(100.4), using an external beam sensing (EBS) system (100.5) that mayinclude beam splitters, mirrors, diffractive gratings etc, which islocated inside the transmitted through fiber tip beam (100.1) and usedto send a portion of the light (100.6) to a sensing module (100.7).

In the approach to laser beam characteristic sensing in FIG. 1B,referred to as fiber-based light tapping, that approach is based onutilization of a fiber-integrated beam splitter (100.11), commonlyreferred to as a fiber tap coupler, that diverts a portion of laser beampropagating in the delivery fiber (100.3) to the tap fiber (100.12) thatroutes it to the sensing module (100.7).

Two examples of possible EBS systems (100.5) are illustrated in FIGS. 2Aand 2B. In the first example, the external beam sensing system iscomposed a small mirror (100.8) that is located inside the transmittedbeam (100.1) and redirects portions of this beam on to the sensingmodule (100.7). The second example of EBS system in FIG. 2B. includesthe lens (100.9) that collimate the transmitted light and a beamsplitter (100.10) that redirect a portion of the transmitted light(100.6) to the sensing module (100.7).

The sensing techniques illustrated in FIGS. 1A and 1B have several majordrawbacks. The external light tapping requires installation andalignment of additional optical elements inside the transmitted beam,which may be difficult or even impossible to implement in practice (e.g.due to constrains on available space, or overall optical system weight).Additionally, these optical elements may noticeably affect (e.g.,perturb or change) laser beam characteristics such as power, intensity,phase profiles, polarization state, and other characteristics (e.g., byintroducing beam obscuration, absorption, optical aberrations). In thecase of high-power fiber laser systems, the external light tapping maybe especially difficult to implement, as the optical elements in thepath of the high-power beam should withstand thermal deformationswithout perturbing laser beam characteristics.

On the other hand, the fiber-based light tapping using fiber-integratedcouplers could provide monitoring of laser light characteristics at thelocation of the fiber tap coupler as illustrated in FIGS. 1A and 1B. Inthis case, the section of the delivery fiber (100.14) located betweenthe fiber tap coupler (100.11) and the delivery fiber tip (100.2) mayintroduce additional changes into the transmitted laser beamcharacteristics which cannot be detected with the fiber tap coupler.Additionally, the existing fiber tap couplers generally only operate atrelatively low (e.g., on the order of tens-to-hundreds of Watts) laserpower and may be inappropriate for monitoring and sensing of laser beamcharacteristics in kW-class fiber-optics systems. The power limitationof the current technology is related to inefficient coupling of the highpower (HP) laser beam to the tap fiber, resulting in generation ofunacceptably high level of heat. From the analysis above, it followsthat development of sensing methods and corresponding devices forin-situ measurement and control of characteristics of laser beam at thedelivery fiber tip, which are non-perturbing, undisruptive, and capableof withstanding kW laser power levels, cannot apparently be basedentirely on the existing technology, and may require differentapproaches. The drawbacks of the current technology can be completely orpartially alleviated with varying implementations of the systems andmethods disclosed herein, which offer desired solutions to the problemsdiscussed above as well as other problems present, whether recognized ornot, in conventional systems.

In particular, implementations of the technology disclosed herein mayprovide a sensing and control system having low insertion loss, thatdoes not affect parameters of the transmitted beam, and that may beapplicable for the use with different fiber types including large modearea (LMA), double-clad fibers, polarization maintaining (PM) and non-PMfibers, and photonic crystal and photonic bandgap fibers. Some or all ofthese objectives could be fulfilled with varying implementations andcombinations of the fiber tip coupling method and devices disclosedherein.

Fiber Tip Couplers for Laser Light Sensing

The disclosed methods and systems may be implemented for in-situ sensingand control of the laser light transmitted through a delivery fiber tipin fiber-optics systems, which may be referred to as fiber-tip-coupler(FTC) sensing methods and systems. An example of an FTC system (200) isillustrated in FIG. 3, and a more detailed view is illustrated in FIG.4.

The FTC system (200) in FIG. 3 is composed of the FTC glass assembly(200.1), with one or several tap fibers (200.2) and sensing module(s)(100.7). The FTC system may be self-contained within a single unit,module, or device, or may be spread across one or more units, modules,or devices. The FTC glass assembly (200.1) in FIG. 3 and FIG. 4 showsthe distal portion (e.g., the last few centimeters) of the deliveryfiber (100.3) with the core (200.3) and cladding (200.4). FIG. 3illustrates the cross section of the FTC assembly along the opticalaxis, while FIG. 4 shows the FTC cross section at the delivery fiber tipplane (100.2). The delivery fiber (100.3) may be coming from afiber-amplifier, fiber laser or other fiber-optics system type, and mayhave a diameter around 400 microns, as in many high-power fiber systems,or any other diameter of commonly used fibers. The delivery fiber(100.3) has attached (e.g. fused) to it an endcap (200.5). The endcap(200.5) has a few mm length and diameter d_(e-c) that may be equal orlarger than the delivery fiber diameter d_(d-f). The endcap (200.5) inFIG. 3 is cleaved at a relatively small (˜4-8 deg.) angle, and could beanti-reflection (AR) coated. Even with the AR coating, a small (<0.1%)fraction of the light (100.1) transmitted through the fiber tip (100.2),referred to as back reflected light (200.6), is reflected off theglass-air interface (200.7) and propagates back inside the endcap(200.5).

The light (200.8) that is transmitted through the endcap (200.5) exitsthe FTC fiber assembly (200.1). The light (100.1) transmitted throughthe fiber tip (100.2), as well as the back reflected light (200.6), andcorrespondingly the transmitted light (200.8) exiting through the endcap(200.5), originate from a laser beam coming from the delivery fiber core(200.3), also referred to as a main beam (200.9). The light componentsthat enter the endcap (200.5) through the delivery fiber cladding(200.4) may be referred to as cladding light (200.10). Note that thecladding light (200.10) has larger divergence compared with the mainbeam (200.9) and may also include residual pump light.

The operation principle of the FTC assembly (200.1) in FIG. 3 and FIG. 4is based on utilization the back reflected light (200.6) for sensing oneof several characteristics of the transmitted light (200.8). Thesecharacteristics may include laser power, polarization and differentialphase, also referred to as piston phase, of the main beam (200.9), powerof the cladding light (200.10), and presence and contribution ofhigh-order modes coming from either the fiber core or cladding. Thesesensing functions of the FTC assembly (200.1) are achieved using aspecially designed angle-cleaved endcap (200.5). The light (200.6) thatis reflected from the glass-air interface (200.7) of the endcap (200.5),propagates back inside the endcap (200.5) to the plane of the deliveryfiber tip (100.2) where it is received by a single or several tap fibers(200.2). In FIG. 4 the tap fibers are shown as including four differentfiber types (200.2A through 200.2D). Both the delivery fiber (100.3) andthe surrounding tap fibers are assembled inside a special shell (200.11)that holds the tap fibers (200.2) and provides coupling of the backreflected light (200.6) into the tap fiber (200.2) tips.

The multi-tap FTC fiber assembly (200.1) in FIG. 4 is composed of fourtap fibers (200.2A-200.2D). For illustration purpose only these tapfibers include: the SM PM tap fiber (200.2A), SM non-PM tap fiber(200.2B), LMA tap fiber (200.2C) and the multi-mode tap fiber (200.2D).These tap fibers (200.2A-200.2D) are attached (e.g. fused) to the backsurface of the endcap (200.5) as shown in FIG. 3, in such a way that theback reflected light (200.9) received by one or many tap fibers (200.2)and delivered to the sensing module (100.7). Note that for in-situsensing of the transmitted light polarization the tap fibers may bePM-type fibers that preserve the polarization state of the backreflected light (200.6).

To optimize coupling efficiency and separate the main beam (200.9) andcladding light (200.10) components of the back reflected light (200.6),the parameters of endcap (200.5) and location of tap fibers (200.2)inside the shell (200.11) may be properly selected through analysisusing conventional ray tracing or wave-optics methods. It may beadvantageous, for efficient sensing of the main beam characteristics,for the position of the tap fiber core to be close to or to coincidewith the center of the back reflected main beam footprint (200.12), asillustrated in FIG. 4. The coordinates of the center depend on thetransmitted beam wavelength and parameters of the fiber endcap (200.5),which may include, for example, endcap refractive index, length, andcleave angle, and may be estimated through ray tracing simulations. FIG.5 provides an example of ray tracing simulations performed for thetransmitted beam wavelength λ=1064 nm, delivery fiber with 19 μm core,silica endcap with diameter d=0.4 mm, length L=3 mm and cleave angle α=5deg. In the ray tracing in FIG. 5, a SM tap fiber with 125 μm diameterand 7 μm core was used. Note that for optimal light coupling the tapfibers may be additionally angle-cleaved and tilted inside the glassshell (200.11).

In the case of delivery fibers that can provide a single-mode or afew-modes operation, such as SM and LMA fibers, the transmitted mainbeam (200.9) may have significantly low divergence compared with thedivergence of the cladding light (200.10). Correspondingly, asillustrated in FIG. 4, the back reflection of main beam results in afootprint (200.12) at the plane of tap fiber tips (100.2) which issignificantly smaller compared to the corresponding footprint (200.13)of the back reflected cladding light. This allows independent detectionof low- and high-divergence components of the transmitted light (200.8),including low- and high-order modes, and cladding and residual pumplight, by arranging the tap fibers in respect to the back reflected mainbeam footprint center (200.12).

For the purpose of illustration, the SM PM tap fiber (200.2A) in FIG. 4is located at the center of the main beam footprint (200.12) and, hence,primarily couples single-mode component of the light transmitted throughthe core (200.3) of the delivery fiber (100.3), thus providingcapabilities for in-situ monitoring of its characteristics (e.g. power,polarization and piston phase). The power level of the light issensitive to the presence or appearance of different modes in the mainbeam (200.9), and may be effectively used for forecasting and preventionof “catastrophic” events in the fiber-optics systems which are commonlyassociated with abrupt laser power drop, such as fiber fusion.

The SM non-PM tap fiber (200.2B) in FIG. 4 is shown as being shifted inrespect to the center of the back reflected main beam footprint(200.12). In this manner, tap fiber light may be used to detectappearance of higher-order modes in the transmitted main beam (200.9).These modes are commonly caused by several factors, includingenvironmental factors such as bending of the delivery fiber, thermal andmechanical disturbances, as well as imperfections in fiber splicers,fiber connectors, defects in fibers and non-linear effects. Theappearance and content of the high-order modes in the transmitted lightmay also be detected using the LMA tap fiber (200.2C) that may befurther shifted off the center of the back reflected main beam footprint(200.12) for better separation of the high-order modes. The LMA tapfiber may have a larger core diameter compared with the SM fiber, whichleads to an increase in the received light power and bettersignal-to-noise ratio (SNR) in high-order modes sensing. For bothcladding and residual pump light sensing, the large-core-diametermulti-mode tap fibers located inside the cladding light footprint(200.13) may have advantages compared with SM and LMA tap fibers, asthey provide better SNR. In FIG. 4 the multi-mode tap fiber (200.2D) isshown as being located outside the back reflected main beam footprint(200.12).

The tap fibers may be encased within a shell (200.11) of the FTCassembly (200.1) using different methods, which may prevent movementduring placement and use and may provide protection to the fibers. Inone implementation of the FTC shell (200.1), disclosed and referred toas a multi-hole glass ferrule (MH-GF), the tap fibers are secured insidea solid block of glass with through holes guiding the delivery (100.3)and surrounding tap fibers (200.2).

In FIG. 3 and FIG. 4, for the purpose of illustration only, the holdingtap-fibers MH-GF shell (200.11) is shown as a solid glass cylindricalglass block (e.g., ferrule) with diameter similar or equal to the endcap(200.5) diameter. The glass ferrule has a set of holes (e.g.,capillaries), one central hole for the delivery fiber (100.3), andperiphery holes for the tap fibers. The glass ferrule with tap fibers isattached (e.g. fused) to the endcap (200.5).

FIGS. 6A and 6B illustrate an example of the MH-GF fiber holder with twoholes for holding a 400 μm SM delivery fiber with 19 μm core and asingle 80 μm multi-mode tap fiber. FIG. 6A shows an exemplary MH-GF(200.11) of diameter 1.0 mm with a central hole for holding of adelivery fiber, and a smaller hole on the bottom for holding a tapfiber. FIG. 6B shows a single-tap FTC assembly based on the MH-GF inFIG. 6B. FIG. 6C shows the results of a single-tap FTC sensorcalibration using the MH-GF of FIG. 6A and 6B. While FIGS. 6A-6C show aspecific example of a delivery fiber and its resulting performance, itshould be understood that delivery fibers having other characteristicsare usable with the systems and methods disclosed herein, with suchvariations being apparent to those of ordinary skill in the art in lightof this disclosure. For example, note that FIGS. 6A-6B show a particularimplementation of a MH-GF shell (200.1), and it will be apparent tothose of ordinary skill in the art, in light of this disclosure, that anMH-GF shell having different characteristics could be made for a desiredapplication. The measured calibration data (200.14) and thecorresponding fitting line (200.15) in FIG. 6C show a correspondence oflight power received through the tap fiber, and light power transmittedthrough the endcap as output laser power. For the FTC sensorcalibration, the single-tap FTC assembly was fiber-spliced to ahigh-power fiber amplifier with the transmitted power ranging from 50 Wto 1000 W, though it should be understood that other operationalcharacteristics are possible and will be apparent to those of ordinaryskill in the art in light of the disclosure herein.

The data represented by the calibration curve (200.15) in FIG. 6C can beused for in-situ sensing of the transmitted through the endcap laserpower with the single-tap FTC sensor. The delivery fiber of FIG. 6A-6Bhad an AR coated 3 mm length endcap of diameter 400 μm with ˜3.4 deg.cleave. After being assembled together the endcap and MH-GF fiber holderwith the delivery fiber and a SM tap fiber were fused together producingthe single-tap FTC optical power sensor.

In some implementations, the FTC assembly (200.1) may be made as anall-glass capillary-integrated multi-tap FTC sensing module (300)referred to as a CIMT sensing module. An exemplary CIMT sensing moduledisclosed herein is illustrated in FIG. 7, FIGS. 8A-8C and FIGS. 9A-9D.

The CIMT sensing module (300) in these figures is composed of a fewcm-long glass capillary (300.1) with a bundle of N (specific for eachdelivery fiber diameter) densely packed tap fibers (200.2) surroundingthe delivery fiber (100.3), as illustrated in FIGS. 7. FIGS. 8A-8Cillustrate the same, for the case of the delivery fiber with diameterd_(d-f)=400 μm and 13 tap fibers of diameter d_(d-t)=125 μm. In the CIMTsensing module (300) the tap fibers may have near identical diameter,but the tap fiber type may be different (e.g. may include differentcombinations of SM PM, SM non-PM, LAM and multi-mode fibers). In apossible CIMT sensing module (300) manufacturing method, the deliveryfiber with a bundle of tap fibers is assembled inside a glass capillary,then collapsed into a solid glass block by heating with CO₂ laser orother means, then cleaved, and spliced to an angle-cleaved endcap thusforming a rigid all-glass module.

FIGS. 8A-8B, and the side view of FIG. 8C, illustrate an exemplary CIMTsensing module (300) made of glass capillary using 400 μm diameterdelivery fiber and a bundle of 13 tap SM PM fibers of 125 μm diameter.FIGS. 8A-8C show a particular implementation of a CIMT sensing module(300), and it will be apparent to those of ordinary skill in the art, inlight of this disclosure, that CIMT sensing modules having differentcharacteristics and manufacturing method could be made. FIG. 8A showsthe cross section of the collapsed into a solid glass block capillarywith the tap fibers after cleaving, and FIG. 8B shows the microscopicview of the CIMT sensing module with the fused endcap of diameter 1.00mm, 3.0 mm length and cleave angle 4.5 deg. In this exemplary CIMTsensing module (300), the endcap was used without AR coating.

The ability to accurately forecast the light power received by each of13 tap fibers in the

CIMT sensing module is illustrated by the plots in FIG. 9A, whichcompares the calibration measurements of the laser power received byeach tap fiber (300.2) with the corresponding numerical simulation data(300.3). The calibration and simulation data were obtained for theexemplary CIMT sensing module (300) in FIG. 8A-8C by fiber-splicing thedelivery fiber of the CIMT module (300) to a high power fiber amplifiersystem with laser power ranging from 50 to 1200 W. For more clarity, thedistribution of the laser power received by the thirteen tap fibers ispresented in FIG. 9B in the form of a polar plot that is superimposedwith a side elevation view of the tap fibers in FIG. 8A. As FIG. 9A and9B indicate, the largest value of received laser power corresponded tothe tap fiber #6. The fiber core of this tap fiber was located near thecenter of back reflected main beam footprint region (200.12), as shownin FIG. 7. Note, that non-zero values of laser power received by tapfibers located at the opposite side from the tap fiber #6 (e.g., tapfibers #1-3 and #10-13 in FIG. 9B) indicate presence of cladding lightcoming from the delivery fiber tip. These tap fibers, located inside thecladding light footprint (200.13), provide sensing of both cladding andresidual pump light. As an illustration FIG. 9B compares the calibrationdata (300.3) of the laser power received by the tap fibers with thecorresponding data (300.4) obtained with intentional bending of thedelivery fiber in order to enhance cladding light contribution in thetransmitted laser power. The bending of the delivery fiber resulted inboth decrease of the main light power received by the tap fiber #6 andincrease in light power received by the tap fibers located inside thecladding light footprint (200.13). The calibration data characterizinglaser power received by the set of 13 tap fibers (200.2) without (300.8)and with (300.4) delivery fiber bending is presented in FIG. 9C. Thecladding light power level could be evaluated by measuringredistribution of the coupling light between the tap fiber in respect tothe calibration curve. Since wavelength of the residual pump light isdifferent from the main beam and cladding light wavelength, the residualpump light may be easily separated from the cladding light usingconventional techniques (e.g., such as those based on a dichroic beamsplitter or narrow band filter).

The ability for the CIMT sensing module (300) to monitor power of thetransmitted laser beam and, thus, be used as an optical power sensor isillustrated using an exemplary dataset in FIG. 9D by the calibrationdata (300.5) and the fitting line (300.6) of the laser power that wasreceived by the tap fiber #6 of the exemplary CIMT sensing module inFIG. 8A-8C. The measurements were performed with gradual increase of thetransmitted through the fiber endcap (200.5) laser power up to 1.2 kW,though it should be understood that other operational characteristicsare possible and will be apparent to those of ordinary skill in the artin light of the disclosure herein.

Methods for Laser Beam Characteristics Sensing and Control with FiberTip Couplers

The sensing module (100.7) in FIG. 3 may be configured in various waysdependent on requirements for laser beam characteristic sensing andcontrol, and the FTC fiber assembly implementation may be based on asingle-tap or multi-tap fibers and utilize either identical or differenttype of tap fibers. The sensing module (100.7) may be composed of asingle or many different sensing nodes. A notional schematic in FIG. 10illustrates possible basic architecture of the FTC sensing and control(FTC-SC) system (400) that includes the sensing module (100.7) comprisedby several sensing nodes. The FTC-SC system (400) includes afiber-optics subsystem (400.1) that may have a single (100.3) or manydelivery fibers that are connected (e.g. using fiber splicing) to theFTC assembly (200.1) with a single or many tap fibers (200.2). These tapfibers deliver a portion of the transmitted light (200.8) to the sensingnodes (400.2) and (400.3). As has been described, received light mayinclude the main beam (200.9) and the cladding light (200.10), and mayinclude the residual pump light component.

For sensing of laser power of the main beam (200.9) or cladding light(200.10), the sensing nodes (400.2) may utilize photo-detectors thattransform the delivered through the tap fibers light to electricalsignals (400.4).

For polarization sensing, the corresponding sensing nodes (400.2) mayadditionally include polarization sensitive elements (e.g. polarizersand polarization beam splitters) that select light components with adesired polarization state. These light components are furthertransformed in the sensing nodes to the electrical signals (400.4).

For residual pump light sensing, the corresponding sensing nodes (400.2)may additionally include optical filters that select light componentswith wavelength of the pump light prior to being converted to electricalsignals (400.4).

For sensing of the differential piston phase of the main beam (200.9),the corresponding sensing node (400.3) has additional optical input thatprovides a reference optical wave that is coherent in respect to themain beam that is received through the tap fibers (200.2). Note that forthe differential phase sensing the tap fiber should be SM fiber. In FIG.10 the optical reference is shown as being delivered to the sensing node(400.3) from fiber-optics subsystem (400.1) through a SM PM fiber(400.5). The sensing node (400.3) may also include a fiber coupler thatcombines both optical inputs, and a polarizer in front of thephoto-detector.

The electrical signals (400.4) from sensing nodes (400.2) and or (400.3)may be either directly used for in-situ monitoring of thecharacteristics of the transmitted through the delivery fiber beam(200.8), or be sent to the signal processing unit (400.6) for theretrieval of the desired characteristics from the sensing data (e.g.retrieval of the polarization state characteristics, content ofdifferent modes in the main beam and cladding light, evaluation of theresidual pump light and coherence length).

Feedback Control of Laser Beam Polarization with Fiber Tip Coupler

The output signals (400.4) of the sensing nodes (400.2), the processedsignals (400.7), or both may be used for programmable or feedbackcontrol of the transmitted beam characteristics, including but notlimited by power, polarization, and piston phase. These signals may beused by the controller (400.8) to generate the control signals (400.9)applied to the corresponding control modules of the fiber-opticssubsystem (400.1) (e.g. laser power controllers, polarization adjusters,phase shifters, optical path adjusters).

A schematic of an exemplary FTC sensing and feedback control system forreal-time stabilization, also referred to as locking, of the transmittedmain beam polarization state is illustrated in FIG. 11. The polarizationlocking system (500) in FIG. 11 includes the fiber optics subsystem(400.1) based on SM non-PM fibers and fiber elements. The examples ofsuch fiber optics subsystems include, for example, the high power singlemode fiber amplifiers and various coherent fiber array laser systemsused in directed energy, laser communications, power beaming,polarization imaging and laser additive manufacturing applications.

The polarization locking system (500) may be used to control and lockpolarization of the transmitted main beam by a fiber-optics system at aspecified polarization state. This polarization locking may be useful,for instance, to obtain an array of identically polarized laser beams incoherent fiber array laser systems based on non-PM fiber amplifiers.Without polarization locking these beams cannot efficiently be phased toachieve maximum laser power density at a remotely located target and ormaterial, a desired goal in many military directed energy applications,as well as other applications. Both polarization locking and phasing ofan array of laser beams may also be advantageous or required inapplications for laser power beaming and free space lasercommunications, for example, to allow for adaptive optics mitigation ofatmospheric turbulence effects.

In an exemplary fiber-optics subsystem (400.1) in FIG. 11, the laserlight from a fiber-coupled laser source (500.1) propagates through acontrollable polarization adjuster (500.2) that could be based, forexample, on such commercial product as PolaStayPOS-002 from GeneralPhotonics Inc., or other fiber-optics devices with similarfunctionalities. The polarization adjuster (500.2) changes thepolarization state of the laser beam in the output fiber (500.3) inresponse to the applied control signals (400.9). The laser beamtransmitted through the polarization adjuster may be further amplifiedby the fiber amplifier (500.4). The delivery fiber (100.3) of the fiberamplifier routes the laser beam to the fiber tip (100.2) located insidethe FTC assembly (200.1).

The FTC assembly (200.1) of the polarization locking system (500) inFIG. 11 utilizes a single or several SM PM tap fibers (200.2) thatpreserve the polarization state of the light received by these tapfibers. For more clarity, only a single tap fiber is depicted in FIG.11. The fiber tip of the tap fiber in the FTC assembly (200.1) islocated inside the footprint (200.12) of the back reflected main beam,as illustrated in FIG. 4 by the SM PM tap fibers (200.2A) and (200.2B).A portion of the main beam is delivered by the tap fiber (200.2) to thesensing node (400.2) of the sensing module (100.7), as shown in FIG. 11.In the sensing node (400.2), the laser light (500.5) is transmittedthrough the tip (500.6) of the tap fiber (200.2). The lens (500.7)focuses the transmitted beam (500.5) at the photo-detector (500.8). Apolarization sensitive element (500.9) (e.g. a polarizer or polarizationbeam splitter) selects a specified polarization component of the mainbeam. The optical power of this light polarization component isregistered by the photo-detector (500.8) and after electronic signalprocessing in the processing unit (400.6), which may include signalamplification, noise filtering, and other processing, is sent to thepolarization controller unit (400.8) that computes the control signals(400.9) applied to the polarization adjuster (500.2). Note that thepolarization sensing node (400.2) in FIG. 11 could be configureddifferently, and may, for example, be based on polarization sensitiveelements integrated into the tap fiber, or may use a combination offiber-integrated and free space optical elements.

For the polarization state locking, the polarization controller unit(400.8) may utilize one or another known control algorithms includingbut not limited to optimization algorithms such as gradient descent andor stochastic parallel gradient descent (SPGD) iterative algorithms thatprovide polarization state locking via optimization (e.g. maximization)of the signal (400.4) registered by the photo-detector (500.8) or thecorresponding electronically processed signal (400.7).

To illustrate, the plots in FIGS. 12A and FIG. 12B show experimentaldata obtained using a polarization locking method such as that describedabove. The fiber-optics system (400.1), operated at 1064 nm wavelength,included a commercially available seed laser, polarization adjuster, andfiber amplifier with 1.0 kW output power level. The 400 μm diameterdelivery fiber (100.3) was assembled inside the exemplary CIMT sensingmodule shown in FIG. 8 and FIG. 9B. The SM PM tap fiber #6, as shown inFIG. 9B, was used as a tap fiber for polarization locking. In thesensing node of the experimental setup, a conventional polarizer wasused to select a linearly polarized component of the main laser beam.The corresponding electrical signal (400.4) was used as the polarizationlocking system performance metric that was maximized using the SPGDcontroller. Both the output laser beam (200.8) polarization angletemporal deviation, shown in FIG. 12A, and the polarization extension(ER) ratio, shown in FIG. 12B, which are indicative of quality ofpolarization locking into a liner polarization state, were measuredusing commercially available polarization analyzing equipment.

The time evolution plots in FIG. 12 show dynamics of both thepolarization angle deviation from the zero angle that was set by thepolarizer, and the polarization extension (ER) ratio, for periods oftime when the feedback control system was OFF (500.10) and periods oftime when the control system was ON (500.11). The achieved level of thepolarization angle deviation not exceeding +/−1.25 deg. and high levelof the polarization extension ratio ˜14.45 dB+/−0.75 dB clearlydemonstrate the effect of the polarization control method and systemdisclosed herein.

Polarization Locking of Multiple Beams in Fiber Array Laser Systems

Locking of polarization states of multiple laser beams may beadvantageous for applications in emerging fiber array laser transmitterand high energy laser (HEL) beam director technologies, includingdirected energy, power beaming, active imaging and remote sensing. Inthese fiber array systems, multiple laser beams are originated in eithera single master oscillator power amplifier (MOPA) fiber-opticssubsystem, or in multiple fiber-optics subsystems. In both cases, thesemulti-channel fiber-optics subsystems may be based on non-PM fiberswhich makes them more efficient, less expensive and able to operate withhigher laser power without being affected by undesired nonlinear effectsin fibers. A disadvantage of the non-PM fiber-based fiber arraytechnology is that polarization states of individual laser beams inthese multi-channel fiber-optics systems are randomly varying. Thisprevents utilization of these systems in the applications that requireidentical or near identical polarization states to perform suchfunctions as (a) adaptive optics compensation of atmospheric turbulenceeffects in directed energy and active imaging systems, (b) adaptiveshaping of laser beam intensity in power beaming and laser additivemanufacturing systems, and (c) controllable change of the polarizationstates in the polarimetric imaging systems.

The schematic in FIG. 13 illustrates a control system architectureusable to provide polarization locking and control of multiple laserbeams in fiber array laser systems without insertion of external opticalelements and sensors to the transmitted laser light. In the multi-beampolarization locking and control system (600) in FIG. 13, multiple laserbeams (200.8) are generated in a fiber-optics subsystem (600.1) that maybe composed of multiple fiber-optics laser modules (400.1), or may beconfigured as a single multi-channel MOPA fiber-optics system. Thegenerated laser beams are routed to the tips of the delivery fibers(100.3) that are assembled inside the FTC assemblies (200.1). Thepolarization locking and control system (600) utilizes a single orseveral SM PM tap fibers (200.2). For more clarity, only a single tapfiber per fiber channel is depicted in FIG. 13. The fiber tips of thetap fibers in the FTC assemblies (200.1) are located inside footprintsof the back reflected main beams, as illustrated in FIG. 4 by the SM PMtap fibers (200.2A) and (200.2B). A portion of each main beam isdelivered by each tap fiber (200.2) to the sensing node (600.2), asshown in FIG. 13. The tap fibers in the sensing node (600.2) arecombined into bundle (600.3) with densely packed fiber tips. The laserlight (600.4) that is transmitted through the tap fiber bundle (600.3)is focused by the lens (500.6) to the photo-detector (500.8). Apolarization sensitive element (500.7) (e.g. a polarizer, polarizationbeam splitter) selects a specified polarization component of the arrayof main beams. The optical power of this light polarization component isregistered by the photo-detector (500.8), and after electronic signalprocessing in the processing unit (400.6), which may include signalamplification, noise filtering, or other processing, is sent to thepolarization controller unit (400.8). The polarization controller unit(400.8) is configured to compute the control signals (400.9) applied tothe polarization adjusters (500.2) of the fiber-optics subsystems(400.1), or to the polarization adjusters that are integrated into eachfiber channel of the MOPA fiber-optics subsystem. Note that thepolarization sensing node (600.2) in FIG. 13 could be configureddifferently, and could, for example, be based on tap fiber integratedpolarization sensitive elements, or a combination of fiber-integratedand free space optical elements.

For polarization state locking, the polarization controller unit (400.8)may utilize one or another known control algorithm, such as the SPGDiterative algorithms, that provide polarization state locking viaoptimization of the signal (400.4) registered by the photo-detector(500.8) or the corresponding electronically processed signal (400.7).

For polarimetric imaging with controllable change of polarization statesfor all laser beams, the polarization sensitive element (500.7) in FIG.13 may provide capabilities for controllable alternation of the lightpolarization component. This change of the selected polarizationcomponent may be performed at a slow time rate compared with thepolarization locking process. The controllable polarization alternationmay be needed to enhance resolution of polarimetric imaging systems usedfor active imaging using the laser light scattered off the target.

Phase Locking of Multiple Beams in Fiber Array Laser Systems

Locking of mutual phases, also referred to as differential or pistonphases, of multiple laser beams may be advantageous or required in fiberarray systems used for directed energy, power beaming, active imagingand remote sensing applications. In such fiber array systems, thetransmitted laser beams originate from a multi-channel MOPA fiber-opticssubsystem. To decrease divergence of the transmitted laser energy, andachieve an increase in laser power density at a remotely located targetthat is illuminated by the fiber array laser transmitter, or toadaptively shape a laser beam on the remote located target, the array offibers may need to be configured to produce stable (e.g., locked) pistonphases for the transmitted beams. Note that phase locking mayadvantageously achieved in a multi-channel MOPA-based fiber arraysystem, referred to as narrow-line or coherent MOPA-based fiber arraysystem, that generates laser beams with nearly identical polarizationstates and optical frequencies.

The schematic in FIG. 14 illustrates a control system architecture(700), that may provide locking and control of piston phases of multiplelaser beams in coherent fiber array laser systems without insertion ofexternal optical elements and sensors to the transmitted laser light.The multiple laser beams (200.8) are generated in a multi-channel MOPAfiber-optics subsystem (700.1). For simplicity and better clarity, inFIG. 14 a coherent MOPA system with only three channels (700.2, 700.3,700.4) is shown. A piston phase of one of channels in the MOPA system isidentified as a reference phase. The feedback phase locking controlsystem is designed to force all piston phases of the transmitted beamsto be either equal the reference phase, or different by a value 2nπradiant (n=1, 2, . . . ), independent on temporal variation of thereference piston phase. For the purpose of illustration only, the fiberchannel (700.2) in FIG. 14 is considered as a reference phase and thepiston phases of two other channels are locked in respect to the pistonphase of the reference channel. The number of fiber channels similar to(700.3) and (700.4) could be one or many.

The laser beams that are generated in the coherent MOPA system (700.1)in the fiber channels are routed by the delivery fibers (100.3) to thecorresponding fiber tips. The end sections of the delivery fibers(100.3) are held inside the FTC assemblies (200.1) with a single orseveral SM PM tap fibers (200.2). The FTC assembly of the referencefiber channel (700.2) may have only a single tap fiber, while FTCassemblies of the other fiber channels may have either a single tapfiber or several tap fibers. For more clarity, in FIG. 14 the FTCassembly of the reference fiber channel (700.2) and the fiber channel(700.4) have a single tap fiber, while the FTC assembly (700.3) has twotap fibers.

The fiber tips of the tap fibers in the FTC assemblies (200.1) arelocated inside footprints of the back reflected main beams, asillustrated in FIG. 4 by the SM PM tap fibers (200.2A) and (200.2B). Aportion of each main beam is delivered by each tap fiber (200.2) to thetap fiber combiners (700.5). The fiber combiners merge laser beamspropagating in two tap fiber coming from different FTC assemblies intothe output fibers (700.6), resulting in their interference. The outputfibers (700.6) are fiber-connected to the corresponding photo-detectors(700.7) as illustrated in FIG. 14. Interference of two laser beamscoming from the tap fibers belonging to different FTC assemblies resultsin modulation of the photo-detector output signals (700.8), whichdepends on mismatch in piston phases of the transmitted beams (200.8) ofthe corresponding fiber-optics channels. Note that specific architectureof tap fibers interconnection with fiber combiners that merge two laserbeams belong to different FTC assemblies may be different. The tap fibercombination in FIG. 14 serves as an illustration only. Other tap fiberinterconnection architectures that provide sensing of mutual pistonphases via interference of various combinations of two beams originatingin different FTC assemblies are also possible. Note that for sensing ofthe two-beam interference-induced modulation of the registered byphoto-detectors signals (700.8), the sensing fiber elements includingtap fibers (200.2) and fiber combiners (700.5) with output fibers(700.6) should preserve polarization states of the laser light receivedby the tap fibers (200.2). A polarization sensitive element (e.g. apolarizer and or polarization beam splitter) may be also used to selecta specified polarization component of the transmitted beams (200.8).

In the phase locking control system (700) in FIG. 14, the signals(700.8) from the photo-detectors (700.7) are routed to the electronicsignal processing units (700.9) that may provide signal amplification,filtering of noise or DC component of the registered signals, or otherprocessing. The processed signals (700.10) are sent to the piston phasecontroller units (700.11) that compute control signals (700.12) appliedto controllable phase shifters (700.13) integrated into fiber channels(700.4) of the MOPA system. These phase shifting elements may be based,for example, on the commercially fiber-integrated LiNbO₃ phase shiftersor similar devices. Note that the two-beam interference sensing in FIG.14 could be configured differently, for using one or another free-spaceinterferometric devices or using combination of fiber-integrated andfree space optical elements to produce two-beam interference.

For the piston phase locking, the controllers (700.11) may utilize acontrol algorithm including such as the SPGD iterative algorithms, oranother algorithm that provides phase locking via optimization of thesignal (700.8) registered by the photo-detector (700.7) or thecorresponding electronically processed signal (700.10).

1. A method comprising: (a) operating a laser light source to transmitlaser light via one or more delivery fibers to a fiber tip couplersensing and control system (FTC-SC); (b) at an endcap interface surfaceof the FTC-SC, transmitting a transmitted portion of the laser lighttowards a target in a first direction; (c) at the endcap interfacesurface, reflecting a reflected portion of the laser light in a seconddirection towards one or more tap fibers; (d) with a sensing module,receiving the reflected portion from the one or more tap fibers; (e)determining at least one characteristic of the transmitted portion basedupon the reflected portion; and (f) providing a set of control signalsto a laser light source controller based upon a desired value for the atleast one characteristic of the transmitted portion and the determinedat least one characteristic of the transmitted portion; wherein the setof control signals are configured to cause the laser light sourcecontroller to modify laser light so that a subsequent determined atleast one characteristics of the transmitted portion substantiallymatches the desired value.
 2. The method of claim 1, wherein the sensingmodule comprises: a first sensing node configured to receive thereflected portion; and (ii) a second sensing node configured to receivea reference optical wave from the laser light source; the method furthercomprising determining, by the sensing module, the at least onecharacteristic of the transmitted portion based upon the reflectedportion and the reference optical wave.
 3. The method of claim 1,wherein the sensing module comprises: a lens configured to receive thereflected portion; (ii) a polarizer element configured to receive thereflected portion from the lens and polarize the reflected portion toselect a specific polarization component; and (iii) a photo detectorconfigured to receive the reflected portion from the polarizer elementand measure characteristics of the reflected portion; the method furthercomprising applying, by a polarization controller, a control algorithmto the set of control signals to provide polarization state locking forthe laser light source.
 4. The method of claim 1, further comprising:(a) transmitting laser light from a set of laser modules of the laserlight source via a plurality of delivery fibers to a plurality of fibertip coupler sensing and control systems (FTC-SCs); (b) with each of theplurality of FTC-SCs, providing reflected light to the sensing module;and (c) with a fiber bundler of the sensing module, receiving andcombining reflected light from the plurality of FTC-SCs prior todetermining the at least one characteristic of the transmitted portionfor each of the plurality of FTC-SCs.
 5. The method of claim 1, furthercomprising transmitting the transmitted portion of the laser light inthe first direction through free space for a transmission distance,wherein the transmission distance is equal to the distance between theendcap interface and the target.
 6. The method of claim 1, furthercomprising: (a) transmitting laser light from the one or more deliveryfibers to the endcap interface surface without transmitting the laserlight through free space; and (b) transmitting laser light from theendcap interface surface to the one or more tap fibers withouttransmitting the laser light through free space.
 7. The method of claim1, further comprising configuring the endcap interface surface with anangle based on the position of the one or more delivery fibers relativeto the one or more tap fibers.
 8. The method of claim 1, furthercomprising configuring the FTC-SC so that the one or more deliveryfibers, the endcap interface surface, and the one or more tap fibershave a permanently fixed position and orientation relative to eachother.
 9. A fiber tip coupler for a sensing and control systemcomprising: (a) an endcap including a light transmitting material and aninterface surface; (b) a laser delivery fiber including a fiber tip andan input tip, wherein the fiber tip is coupled to the endcap at a firstposition and the input tip is configured to couple with a laser lightsource; (c) a tap fiber including a tap fiber tip and an output tip,wherein the tap fiber tip is coupled to the endcap at a second positionand the output tip is configured to couple with a laser sensing system;wherein the interface surface is configured to have an angle based onthe first position and the second position, and wherein the angle isselected to cause laser light from the fiber tip to be separated into:(i) a transmitted portion that enters free space via the interfacesurface, and (ii) a reflected portion that reflects off the interfacesurface and strikes the tap fiber tip.
 10. The fiber tip coupler ofclaim 9, wherein the light transmitting material comprises a glassmaterial.
 11. The fiber tip coupler of claim 9, further comprising ashell, wherein: (i) the shell includes a plurality of capillaries, (ii)the laser delivery fiber and the tap fiber tip are each at leastpartially fused within capillaries of the plurality of capillaries sothat the fiber tip and the tap fiber tip are at a surface of the shell,and (iii) the shell is coupled to the endcap.
 12. The fiber tip couplerof claim 9, wherein the tap fiber tip is fused to the endcap at anon-orthogonal tap angle, wherein the non-orthogonal tap angle isselected based on the angle of the interface surface, the firstposition, and the second position.
 13. The fiber tip coupler of claim 9,wherein the first position, the second position, and the angle of theinterface surface are selected based upon a ray tracing simulation or awave-optics method.
 14. The fiber tip coupler of claim 9, wherein thetap fiber is configured as a single mode polarization maintaining tapfiber, the fiber tip coupler further comprising: (a) a single modenon-polarization maintaining tap fiber fused to the endcap at a thirdposition; (b) a large mode area tap fiber fused to the endcap at afourth position; and (c) a multi-mode tap fiber fused to the endcap at afifth position; wherein the second position, third position, fourthposition, and fifth position are each within a beam footprint of thereflected portion.
 15. The system of claim 9, wherein: (a) the firstposition at which the fiber tip is fused to the endcap is substantiallycentrally positioned on a back surface of the endcap; (b) the secondposition at which the tap fiber tip is fused to the endcap, as well as aplurality of positions at which a plurality of tap fiber tips are fusedto the endcap, including the second position, are arranged to surroundthe first position; and (c) each of the plurality of positions is oneof: (i) positioned entirely within a main beam footprint of thereflected portion; (ii) positioned partially within the main beamfootprint and partially within a cladding light footprint of thereflected portion; (iii) positioned entirely within the cladding lightfootprint; (iv) positioned partially within the cladding light footprintand partially outside of the reflected portion; or (v) positionedentirely outside of the reflected portion.
 16. A method comprising: (a)operating a laser light source to transmit laser light to a fiber tip ofa delivery fiber, wherein the fiber tip is coupled to an endcap at afirst position of the endcap; (b) receiving, by the end cap, the laserlight, wherein the endcap includes a light transmitting material and aninterface surface, and wherein the interface surface is configured toreceive and separate laser light from the fiber tip into: a transmittedportion that enters free space via the interface surface, and (ii) areflected portion that reflects off the interface surface; and (c)receiving the reflected portion with one or more tap fibers that arecoupled a second position of the endcap.
 17. The method of claim 16,further comprising (a) providing the reflected portion to a sensingmodule; (b) measuring at least one characteristic of the reflectedportion using the sensing module; and (c) determining at least onecharacteristic of the transmitted portion based upon the at least onecharacteristic of the reflected portion.
 18. The method of claim 17,further comprising: (a) generating a control signal based upon a desiredvalue for the at least one characteristic of the transmitted portion andthe determined at least one characteristic of the transmitted portion;and (b) providing the control signal to a laser light source controller;wherein the control signal is configured to cause the laser light sourcecontroller to modify the laser light produced by the laser light sourceso that a subsequent determined at least one characteristic of thetransmitted portion substantially matches the desired value.
 19. Themethod of claim 16, further comprising selecting an angle of theinterface surface based upon the first position and the second positionof the endcap.
 20. The method of claim 16, further comprising fusing thefiber tip to the endcap at the first position and fusing the one or moretap fibers to the endcap at the second position to provide a permanentlyfixed position and orientation relative to the interface surface.